1. Field of the Invention
The present invention relates to a method and an apparatus for forming a single optical disc substrate by bonding optical disc substrates together and a method for supplying a liquid material.
2. Description of the Related Art
When optical disc substrates are bonded together by an optical disc bonding apparatus using a liquid adhesive, it is important to ensure that no void exists in the adhesive layer after the bonding of the substrates. Therefore, various proposals have heretofore been considered, but all proposals have been unsatisfactory in that voids having a diameter of about 0.1 mm or larger, minute voids having a diameter of about 0.05 to 0.1 mm, or a mixture of these voids are formed between the optical disc substrates.
As a method for significantly minimizing such a problem, the applicant of the present invention has applied the following invention, described in Japanese Patent Application No. Hei 10-257530. The invention will be described with reference to FIG. 8. Of two optical disc substrates A and B, a ring-shaped adhesive liquid film Ta is formed on the upper bonding surface of the lower optical disc substrate A. On the bonding surface of the upper optical disc substrate B, a plurality of dot-shaped adhesive liquid films Tb are formed in a circular shape having a diameter slightly larger than that of the ring-shaped adhesive liquid film Ta. Thereafter, the two optical disc substrates A and B are brought close together with the bonding surfaces thereof facing each other, and the two optical disc substrates A and B are joined together bringing the ring-shaped adhesive liquid film Ta into contact with the dot-shaped adhesive liquid films Tb. Then, the two optical disc substrates A and B are spin-processed to spread out the adhesive liquid film Ta and the adhesive liquid films Tb. The excess adhesive is spun off from the substrates, and an adhesive layer having a uniform film thickness is formed between the optical disc substrates A and B.
In this method, by properly bringing the ends of the dot-shaped adhesive liquid films Tb, which are formed in a circular shape on the upper optical disc substrate B, into contact with the rim of the ring-shaped adhesive liquid film Ta formed on the lower optical disc substrate A, the development of particularly minute voids at the moment when these liquid films make contact with each other can be prevented. Further, since air between the liquid films is expelled when the contact portions between the liquid films expand over the entire liquid films, the occurrence of voids is lower at these points.
However, since it is still extremely difficult, even by this method, to make the contact area sufficiently small at the moment when the adhesive liquid film Ta and the adhesive liquid films Tb contact each other, the development of minute voids cannot be totally prevented. Further, voids may develop when the adhesive liquid film Ta or the adhesive liquid films Tb make contact with the opposing optical disc substrate B or A.
Further, since voids may also be formed when the adhesive liquid film Ta or the adhesive liquid films Tb are formed by supplying a liquid adhesive from an adhesive-supplying nozzle (not shown) onto the lower optical disc substrate A or onto the upper optical disc substrate B, the formation of voids during this process also needs to be prevented.
The present invention has been invented in view of the above circumstances. The object of the present invention is to provide a method and an apparatus for bonding optical disc substrates together, which rarely or never generate voids between the optical disc substrates when a liquid adhesive is supplied onto the optical disc substrate or the like, or when the two optical disc substrates or the like supplied with an adhesive are bonded together.
The present invention relates to a method for bonding two optical disc substrates together which comprises the steps of joining the optical disc substrates together with an adhesive and curing the adhesive, in which the adhesive is supplied onto the optical disc substrate by an electric field formed between an adhesive-supplying nozzle, for supplying the adhesive onto the optical disc substrate, and the optical disc substrate, and the two optical disc substrates are then joined together and spun by a spinning process.
When the adhesive is supplied onto one of the two optical disc substrates in the shape of a ring, the other optical disc substrate may not have the adhesive applied thereon at all, may have the adhesive film formed over almost the entire surface, or may have the adhesive formed into dots at a relatively small spacing in a circular shape.
When the adhesive is supplied as dots at a relatively small spacing in a circular shape on one of the two optical disc substrates, the other optical disc substrate may not have the adhesive applied thereon at all or may have the adhesive film formed over almost the entire surface.
The present invention also relates to an apparatus for bonding optical disc substrates together by joining the two optical disc substrates together with an adhesive and curing the adhesive, which comprises an adhesive-supplying nozzle for supplying the adhesive onto the optical disc substrate, an electrode means placed in contact with or in the vicinity of the surface of the optical disc substrate which is opposite the surface which faces the adhesive-supplying nozzle, and an electric power supply for generating an electric field between the electrode means and the adhesive-supplying nozzle.
The adhesive-supplying nozzle comprises a single nozzle or two nozzles separated from each other by almost 180 degrees, is placed over the optical disc substrate nearly perpendicular thereto with its tip(s) pointing downward, and forms a ring-shaped adhesive liquid film on the optical disc substrate which spins relative to the nozzle(s).
The adhesive-supplying nozzle may comprise, for example, a plurality of nozzles placed at an approximately uniform spacing in a circular shape, may be placed under the optical disc substrate nearly perpendicular thereto with their tips pointing upward, and may supply dot-shaped adhesive liquid films onto the underside of the optical disc substrate.
In this case, the electric field generated by the electric power supply may be an alternating-current or a direct-current electric field. However, the alternating-current electric field is preferable.